Electroplating baths for depositing tin, lead, or their alloys have been used for many years in electroplating equipment. High speed electroplating equipment and processes are well-known in the industry and generally consist of directing the work to be plated into the electroplating cell from one end, allowing the work to proceed through the electroplating cell and exit thereafter the cell at the other end. The electroplating solution is removed or overflows the electroplating cell into a reservoir and the solution is pumped from the reservoir back into the electroplating cell to provide vigorous agitation and solution circulation. Many variations of these electroplating cells can exist, but the general features are as described.
There are a number of desirable features that the electroplating solution should possess for improved operation in this type of equipment or processing, as follows:
1. The solution must be able to electroplate the desired deposit at the high speeds required.
2. The solution must deposit tin of sufficient purity and grain characteristics to enable a bright reflective deposit to be obtained by reflow, i.e., by heating the tinplate above the melting point of tin and quenching the tinplate into water.
3. The solution should be stable and the additives must withstand exposure to the strong acid solution as well as to the introduction of air which would take place as a result of the vigorous solution movement in high speed plating machines.
4. The solution should remain clear and free from turbidity, even at elevated temperatures such as 120.degree.-130.degree. F. or higher. Due to the high current densities involved and relatively low solution volumes, these baths tend to heat up in high speed electroplating equipment until the solution reaches equilibrium at an elevated temperature. The additives used must be of a type that will not turn the solution turbid at such elevated temperatures.
5. Because of vigorous solution movement and solution mixing with air, there is a strong tendency to produce a foam which is detrimental to the electroplating process. Under extreme conditions, this foam can build up in the reservoir tank with resultant overflow onto the floor, thereby losing a large quantity of solution to the waste stream. Foam can also interfere with the operation of the pump that is being used to generate agitation. Arcing between the anode and cathode is also possible due to the presence of foam. Because of these problems, the additives used should not generate foam in the plating equipment.
Many electrolytes have been proposed for electroplating tin, lead, and tin/lead alloys and one of these is described in U.S. Pat. No. 4,701,244. This patent discloses the electroplating of tin, lead or tin/lead alloys from lower alkyl sulfonic acid baths which contain brightening additives as well as many wetting agents of various types. Surfactants claimed to be useful are betaines, alkylene oxide polymers, imidazolinium compounds, quaternary ammonium compounds, ethylene oxide derivatives of amines, phosphonates, amides and many others. U.S. Pat. No. 4,662,999 discloses an electroplating bath for electrodeposition of tin, lead, or tin/lead alloys from alkane or alkanol sulfonic acid baths that also contain surfactants plus other additives. In this patent, the surfactant can be non-ionic, cationic, anionic or amphoteric. A great many examples are given for the various types of surfactants and the patent enumerates a large number of the various types of wetting agents which can be used.
U.S. Pat. No. 4,673,470 describes a tin, lead, or tin/lead alloy plating bath based upon an aliphatic or aromatic sulfocarboxylic acid. Instead of the alkene or alkanol sulfonic acids disclosed in previous patents, this patent includes a carboxylic acid radical in the organic sulfonic acid compound. The electroplating baths described contain brightening agents plus a surface active agent, with particular emphasis on those surface active agents that are non-ionic. A very broad group of non-ionic surface active agents as described as being useful, and many different wetting agents are recited.
The electrolytic tinning of steel strip to produce tinplate is well known in the art. A detailed summary of this process is described in a book by W. E. Hoare, et al., entitled "The Technology of Tinplate," St. Martin's Press, New York, 1965. Typical plating units are described in Chapter 8, pages 223-252.
Although various types of alkaline and acid electrolytes have been utilized to deposit tinplate, current technology primarily utilizes the sulfate based electrolytes which also contain phenolsulfonic acid for this application. Waste disposal from these baths is difficult, since they contain poisonous materials that are harmful to the environment. Other baths contain fluorides, borates and other environmentally unsafe components.